1. Field of the Invention
The present invention relates to an optical inspection method and an optical inspection apparatus, irradiating a light beam onto an object to be inspected, such as a thin film substrate, a semiconductor substrate or a photomask, for detecting a foreign matter or a defect on the object to be inspected, and in particular an optical inspection method or an optical inspection apparatus, which enhance the detectability for a micro foreign matter or a micro defect.
2. Related Art
In a production line for a thin film substrates such as a semiconductor substrate, inspection for a foreign matter sticking to an outer surface of the thin film such as a semiconductor substrate has been carried out in order to monitor a condition of rising dust of a manufacturing apparatus. For example, it is required to detect a micro foreign matter or a micro defect, not greater than about 10 nm on the outer surface of a semiconductor substrate before formation of a circuit pattern thereon. Conventionally, in a technology for detecting a micro defect on the outer surface of an object to be inspected, such as a semiconductor substrate, as disclosed in U.S. Pat. No. 5,798,829, a focused laser beam is statically irradiated on to the outer surface of the semiconductor substrate (an area which is defined by the laser beam irradiated at this time onto the outer surface of the semiconductor substrate will be hereinbelow referred to as “illumination spot”), and scattered light from a foreign matter which possibly sticks to the semiconductor substrate that is fed on rotation or translation is detected so as to detect a foreign matter or a defect over the entire surface of the semiconductor substrate. An ellipsoidal mirror is used for detecting the scattered light, which is arranged in such a way that the primary focal position of the ellipsoid thereof is set at a detecting position on the semiconductor substrate while the secondary focal position thereof is set at an light receiving surface of a light receiving element, and the scattered light from a foreign matter is collected over a wide solid angle in order to detect even a micro foreign matter.
In the inspection apparatus for a foreign matter and a defect, as stated above, a movable stage for carrying an object to be inspected, capable of both rotation for main scanning and translation for sub-scanning in combination is usually used in order to, in general, spirally scan the object to be inspected. In a method for controlling the drive of the stage as mentioned above, there may be mainly carried out either constant linear velocity scanning with which an object to be inspected is scanned at a substantially constant linear velocity over its substantially entire surface, or a substantially constant angular velocity scanning with which the object to be inspected is scanned at a substantially constant angular velocity over its substantially entire surface. Above all, there may be often used the latter scanning, that is, the constant angular velocity scanning, in the case of the inspection is important.
If the inspection apparatus for a foreign matter and a defect according to the above-mentioned conventional technology, is used in the case of the constant angular velocity scanning, the substantially entire surface of the object to be inspected can be inspected at a speed nearly equal to the maximum speed of the movable stage for an object to be inspected. Accordingly, the inspection time can be shortened. Meanwhile, there has been caused such a problem that the detectability is different between the inner peripheral part and the outer peripheral part of the object to be inspected, and in particular, the detectability is lowered at the outer peripheral part of the object to be inspected. This is because the linear velocity is higher in the outer peripheral part than in the inner peripheral part in the constant angular velocity scanning so that the time for which a foreign matter or a defect passing through an illumination spot produces scattered light, diffracted light or reflected light becomes shorter than in the outer peripheral part. It has been known that scattered light, diffracted light or reflected light generated from a foreign matter or a defect is theoretically proportional to the intensity of illumination light, that is, the illumination intensity of an illumination spot. Since the “net signal value” of a detection signal of scattered light, diffracted light or reflected light is determined in general “(Intensity of Light to be detected)×(Duration Time of Produced Light)”, the “net signal value” is less in the outer peripheral part so as to lower the detectability in such a condition that the illumination intensity is constant in the illumination spot.
This problem can be solved by lowering the linear velocity in the outer peripheral part. However, it causes lowering of the inspection throughput. There could be considered such a solution that the intensity of the illumination spot itself is sufficiently increased in order to enhance the detectability even in the outer peripheral part (the detectability in the inner peripheral part becomes, of course, extremely higher in this case). However, should the intensity of the illumination spot be higher, the energy generated by a laser beam to be irradiated would be excessively large, and accordingly, the temperature of the object to be inspected, in particular, the temperature in the vicinity of the outer surface thereof would be greatly increased, resulting in such a risk that the material or the structure of the object to be inspected would be damaged. Thus, increasing of the intensity of the illumination spot should be limited, and accordingly, the illumination intensity cannot be increased exceeding the limitation.